The warm encounter in Marseille with the president of the Provence-Alpes-Cote d'Azur region represents a potential blow to rivals on the right - especially official conservative candidate Francois Fillon - but it but could raise eyebrows among leftwing voters thinking of backing Macron.

Macron told reporters that Estrosi, a close ally of ex-president Nicolas Sarkozy, was a "bulwark" against far-right leader Marine Le Pen, the 39 year-old's main opponent in the election.

Macron has said he wants to bridge the right-left divide of mainstream French politics. Estrosi is from a wing that is much more conservative than the Socialist government under which the 39-year-old served as economy minister until last year.

"We are responsible for, and concerned about, the interests of France," Estrosi told reporters at the meeting.

Macron and Le Pen are neck-and-neck at the top of the opinion polls ahead of the first round of presidential voting on April 23. Fillon is in third place. Such an outcome would put Le Pen and Macron through to a second round runoff on May 7 which the polls say Macron would win.

Macron said good citizens of France's republic "know what the dangers are for the republic and where the true enemies are".

In a comment on Twitter he added: "The priority is to fight the National Front. Christian Estrosi has been the bulwark against them in this region. I know my priorities".

Macron later held a rally in the Mediterranean port city.

Despite his close affiliation to Sarkozy, Estrosi has been among those within The Republicans party calling for Fillon to step down as a scandal over allegations his wife did no real work for her pay as his taxpayer funded parliamentary assistant hurt his campaign.

BURKINI FACTOR

In the 2015 regional elections, Alpes-Haute Provence Socialist party deputy Christophe Castaner, now a backer of Macron, withdrew the Socialist party list from the second round of voting, asking his voters to back Estrosi to keep the National Front from power.

However, there is no love lost between Estrosi and the mainstream left in France, and Macron's move may cause consternation among his followers there.

Security, immigration and concerns France is losing a sense of identity are hot-button issues in Estrosi territory, home to a large muslim population, and where Le Pen's plans to cut immigration and ban religious clothing in public places resonate.

Estrosi was among conservative politicians who backed calls for a ban on the body-covering burkini swimsuit that some muslim women wear on the beach - proposals vehemently rejected by the Socialist government and knocked down by French courts.

Asked about the Macron-Estrosi meeting, the Fillon team called it opportunistic.

"Macron is hunting on the territory of the right. But he is not the candidate of the right," said Fillon spokeswoman Annie Genevard.